1. Technical Field
The present invention relates to a photodetector receiving object light and ambient light as incident light, an electro-optical device having the photodetector, and an electronic apparatus having the electro-optical device.
2. Related Art
Electronic apparatuses such as personal computers and mobile phones having an electro-optical device mounted thereon as a display device are generally used under a variety of ambient conditions in which environment light varies in intensity. Accordingly, if driving conditions of electro-optical devices can be adjusted in response to the variation in intensity of the environment light, it is possible to improve image quality and to reduce power consumption. For example, in transmissive and transflective liquid crystal display devices, a backlight unit is disposed on the back side of a liquid crystal panel and light emitted from the backlight unit is modulated by the liquid crystal panel. In such a liquid crystal display device, a large amount of power is consumed by the backlight unit. However, if the liquid crystal display device is provided with a photodetector and the intensity of light emitted from the backlight unit adjusted in accordance with the intensity of the environment light, it is possible to reduce the power consumption. In addition, if the photodetector is formed on an element substrate of the liquid crystal display device, it is also possible to reduce cost for parts of the liquid crystal device. However, when an optical sensor is provided on the element substrate, not only object light to be detected but also light from the backlight unit are incident on the optical sensor.
A configuration is disclosed in JP-A-2006-118965, in which a primary sensor outputting a first current corresponding to the intensity of incident light and a sub-sensor outputting a second current corresponding to the intensity of ambient light are electrically connected in series with each other and a differential current between the first current and the second current, which is output from a node between the primary sensor and the sub-sensor, is detected by the use of a capacitor.
However, in the configuration described in JP-A-2006-118965, even if the primary sensor and the sub-sensor have the same photoelectric conversion characteristic, the intensity of the ambient light cannot be precisely detected in a certain range of ambient temperature. As a result, there is a problem in that the electro-optical device cannot be driven under the optimum conditions.
The inventor of the invention has tried to ascertain why the intensity of environment light (object light) cannot be precisely detected in a certain range of ambient temperature when the primary sensor and the sub-sensor have the same photoelectric conversion characteristic, and has arrived at the following conclusions.
First, the inventor studied the relationship between the ambient temperature and the current output from a PIN diode at the time of applying a voltage to photodiodes serving as the primary sensor and the sub-sensor, and obtained the results shown in FIGS. 12A and 12B. Here, the photodiodes used in the study were PIN photodiodes, each of which has an N-type region, an intrinsic region, and a P-type region in a polysilicon layer.
The current-voltage characteristic of the photodiode is shown in FIG. 12A. With the variation in ambient temperature, the current-voltage characteristic of the photodiode indicated by a solid line is changed to the characteristics indicated by a dashed line, a chain double-dashed line, and a dotted line in this order as the ambient temperature varies. The change in the current-voltage characteristic is attributable to dark current which will be described below with reference to FIG. 12B. Here, dark current means a current which flows across a diode when light is prevented from impinging on a photodiode. Dark current is attributable to the temperature of an intrinsic region. Moreover, in the PIN photodiode using a polysilicon layer, unlike a bulk-silicon-based PIN photodiode in which an N-type region, an intrinsic region and a P-type region are stacked, the N-type region, the intrinsic region, and the P-type region are laterally arranged. Because of the lateral arrangement, a junction area is narrow and dark current is thus liable to occur.
FIG. 12B shows the relationship between the ambient temperature and the current output from a photodiode at the time of applying a reverse bias voltage of −4V to the photodiode serving as the primary sensor or the sub-sensor. In FIG. 12B, the solid line L1 indicates apparent photocurrent (the sum of true photocurrent and dark current) which flows when 500 lx of light is incident on the photodiode at ambient temperature, the dotted line indicates a current including only the dark current which flows under a condition that light is blocked so as not to be incident on the photoconductor. As shown in FIG. 12, the apparent photocurrent merely changes at a temperature of 25° C. (room temperature) or less but increases at a temperature higher than 50° C. On the other hand, the dark current is so small as to be negligible at a temperature of 25° C. (room temperature) or less, but increases as the temperature rises. The magnitude of the dark current becomes equal to that of the apparent current at a temperature higher than 50° C. That is, in the photodiode, the dark current prevails in the apparent photocurrent with the rise in ambient temperature. The dark current also increases with an increase in applied voltage.
Accordingly, in the photodetector disclosed in JP-A-2006-118965, when impedance of the primary sensor decreases due to the light radiation under a condition of a high temperature, the balance of a reverse bias voltage is broken. As a result, the dark current markedly affects the differential current and it is thus impossible to precisely detect the intensity of object light.